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The Transport of Nutrients Across the Cell Membranes

Nutrients can move in and out of the body cells by mechanisms described below.

1. Passive Diffusion

Passive diffusion (from the Latin diffundere = to scatter, spread out) is the free movement of nutrients across the cell membranes from the side with high to the side with low nutrient concentration. This is how certain nutrients, such as glycerol, short- and medium-chain fatty acids, and ethanol move from the bowel across the intestinal lining cells into the blood and from the blood to the tissue cells.

2. Facilitated Diffusion

Facilitated diffusion (from the French faciliter = to render easy) is the movement of nutrients with the help of certain carriers [1].

Mineral ions, such as sodium [Na+], potassium [K+], calcium [ Ca++] and chloride [ Cl-], and larger molecules, such as glucose, amino acids and long-chain fatty acids, move in and out of the cells down their concentration gradient with the help of substance-specific carriers (proteins), which form channels or “pores” in the cell membranes.

Transport of glucose into the tissues is facilitated by the hormone insulin.

3. Osmosis

Osmosis (from the Greek osmos = push) is the movement of water across the cell membranes from the solution with lower concentration (hypotonic solution) to the solution with higher concentration of solutes (hypertonic solution). Osmosis is the main mechanism of water distribution in the body [1].

When you drink usual beverages, like water and fruit juices, the fluid in your intestine will become less concentrated (hypotonic) in relation to the blood plasma, so it will move across the intestinal wall into the blood, by the principle of osmosis.

Osmosis can also cause problems:

When a person with lactose intolerance drinks a lot of milk, the unabsorbed lactose will build up in the intestinal fluid, which will become hypertonic in relation to the fluid in the intestinal wall, so the fluid will start to move from the intestinal wall into the intestine and thus cause diarrhea.

When a person drinks a large amount of water in a short time and consumes no or very little salt, the water absorbed from the intestine into the blood plasma will make blood plasma less concentrated (hypotonic) in relation to the fluid in the brain, so the water will move from the blood into the brain cells resulting in brain swelling. This is called water intoxication.

4. Cotransport

In cotransport, one nutrient helps to transport another nutrient across the cell membrane.

Sodium, which moves into the cells down its concentration gradient, helps glucose to enter the cells up its concentration gradient. This is called sodium-glucose cotransport [2].

5. Active Transport

Active transport is the movement of nutrients across the cell membranes with the help of pumps — the most known is the sodium/potassium pump [1]. Active transport requires energy produced by breakdown of highly-energetic molecules of adenosine triphosphate (ATP).

Sodium/potassium (Na+/K+) pumps in the cell membranes pump the sodium ions out and the potassium ions into the cells thus maintaining high sodium concentration outside and low sodium concentration inside the cells, and high potassium concentration inside and low potassium concentration outside the cells [1].

6. Endocytosis

In endocytosis, the cells engulf nutrients from the blood. Cholesterol and some other molecules enter the cells by endocytosis [3].

The Brain-Blood Barrier

The blood-brain barrier (BBB), which consists of the lining (endothelium) of the brain arteries, does not allow certain nutrients and drugs to enter the brain from the blood.

Nutrients that can readily cross the BBB:

  • Water [7-p.122]
  • Minerals [4]
  • Vitamins [4]
  • Glucose, mannose, galactose [7-p.127]
  • Amino acids (most) [4]
  • 5-hydroxytryptophan [13]
  • Fatty acids:
    • Essential [5]: alpha-linoleic, linolenic
    • Non-essential [8]: docosahexaenoic acid (DHA), oleic, palmitic
  • Ketones (produced from fatty acids) [4]
  • Ethanol [4]
  • Phytonutrients: resveratrol [11,12], curcumin [14]

Nutrients that can cross the BBB slowly/in small amounts:

  • Fructose [4]
  • Amino acids alanine, glycine, proline and γ-aminobutyric acid (GABA) [4]

Nutrients that cannot cross the BBB:

  • Disaccharides (sucrose, maltose, etc.)
  • Oligosaccharides, such as raffinose
  • Polysaccharides, such as starch
  • Triglycerides
  • Some nonessential fatty acids [6]
  • Cholesterol [6,9]

It is not yet known if these nutrients can cross the BBB:

  • Polyphenols (flavonoids, tannins) [10,12]

Transport of Nutrients across the Placenta

Nutrients that can cross the placenta [15]:

  • Water
  • Minerals
  • Vitamins
  • Glucose
  • Amino acids
  • Fatty acids (but not triglycerides) [16]
  • Cholesterol
  • Ethanol

  1. Transport across cell membranes  RCN.com
  2. Wright EM et al, 2011, Biology of Human Sodium Glucose Transporters  Physiological Reviews
  3. Endocytosis  RCN.com
  4. Laterra J et al, 1999, Blood-brain barrier  National Center for Biotechnology Information
  5. Hamilton JA et al, 2007, A model for fatty acid transport into the brain  PubMed
  6. Edmond J, 2001, Essential polyunsaturated fatty acids and the barrier to the brain: the components of a model for transport  PubMed
  7. Neuwelt EA, 1989, Implications of the Blood-Brain Barrier and Its Manipulation: Volume 1 Basic Science Aspects, pp. 119-127
  8. Pan Y et al, 2015, Fatty Acid-Binding Protein 5 Facilitates the Blood–Brain Barrier Transport of Docosahexaenoic Acid  ACS Publications
  9. Vitali C et al, 2014  Cardiovascular Research
  10. Schaffer S et al, 2012, Do polyphenols enter the brain and does it matter? Some theoretical and practical considerations  PubMed Central
  11. Turner RS et al, 2015, A randomized, double-blind, placebo-controlled trial of resveratrol for Alzheimer disease
    PubMed Central
  12. Rege SD et al, 2014, Neuroprotective effects of resveratrol in Alzheimer disease pathology  PubMed Central
  13. 5-Hydroxy-DL-tryptophan  PubChem
  14. Mishra S et al, 2008, The effect of curcumin (turmeric) on Alzheimer’s disease: An overview  PubMed Central
  15. Brett KE et al, 2014, Maternal–Fetal Nutrient Transport in Pregnancy Pathologies: The Role of the Placenta
    PubMed Central
  16. Cetine I et al, 2009, Long chain fatty acids and dietary fats in fetal nutrition  PubMed Central

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